Considerable work has been expended in improving techniques for bonding integrated circuit chips to conductive circuit patterns supported on a substrate. One such method uses an anisotropically conductive adhesive between bonding pads on the substrate and matching bonding pads of the integrated circuit chip. The adhesive is typically an insulative polymer containing conductive particles that simultaneously contact the pad of the chip and the pad of the substrate to provide interconnection. The conductive particles do not provide any significant lateral or horizontal conduction; since they transmit current only in the vertical direction between substrate and device bonding pads, the conduction is referred to as "anisotropic." The adhesive of the polymer is cured after mounting of the chip on the substrate which thereafter provides a permanent structural bond in addition to a conductive interconnection. High component density and bonding pad densities can be accommodated by conductive adhesives, and their use generally reduces assembly costs.
Anisotropic conductive adhesives are particularly promising for reducing the cost of bonding chips having a high density of bonding pads spaced only a very small distance from the bonding pads of the substrate. For example, if they could be used for interconnecting bonding pads spaced only about fifteen microns from substrate bonding pads, with the dimensions of the bonding pads being on the order of only one hundred microns on a side, assembly costs could be considerably reduced compared to other methods, and the losses due to inductance through the connections could be reduced because of the close spacing. The copending patent application of N. R. Basavanhally and B. H. Cranston, Ser. No. 07/735,231, filed Jul 24, 1991, is directed to a method for compensating for differences in conductive particle diameter when interconnecting bonding pads of such small dimension.
Even with the improvement of the Basavanhally et al. patent application, conductive interconnection has not proven to be as reliable as would be desired. The distribution of the particles throughout the conductive adhesive is sometimes not reliable. If the particles are too thinly distributed in an area, they may not interconnect matching bonding pads; if they are too thickly distributed, spurious lateral conductive paths may be established which can lead to short circuit in the structure.
Accordingly, there has been a continuing long-felt need for conductive adhesive techniques that can provide reliable conduction and bonding, especially for bonding small-area bonding pads of integrated circuits with matching bonding pads of a substrate.